In general, the conventional backlights for the liquid crystal display include such types as electroluminescent (EL) backlight, cold-cathode tube backlight, and light-emitting diode (LED) backlight, etc. The EL backlight generates electromagnetic interference while emitting light and its luminance is also weak. The cold-cathode tube backlight demands high-voltage excitation, so an inverter must be provided to trigger it. Due to spatial restriction, the cold-cathode tube backlight is not suitable to be applied in a small-sized liquid crystal display device. The LED backlight usually employs a white light source containing three colors of red, green and blue, when the white light passes through sub-pixels of the liquid crystal display device, only light of one color can pass through with a large transmittance while light of the other two colors are absorbed by a color filter film. As shown in FIG. 1, each pixel of the liquid crystal display device includes sub-pixels in three colors of red, green and blue; when a white light source is adopted, only light of one color passes through the corresponding sub-pixels, that is, individual sub-pixels emits red, green or blue monochromatic light, respectively. As a result, most of the light is lost when passing through the liquid crystal display device, thereby the light utilization ratio is low.